(physics) the unit of exposure to X-rays or gamma rays. Whereas a curie measures the amount of activity in radioactive material itself, a roentgen is a measure of the radiation generated by that material. Named afterGermanphysicist Wilhelm Conrad Rontgen (1845 - 1923) who discovered X-rays in 1895. Abbreviation r. —All About Radiation Glossary. Final approval 27.9.89